During operation, blades of turbomachines, in particular turbine blades, are subjected to very high material loading and to corresponding high wear. For this reason, the blades are replaced with new blades within the framework of maintenance work, normally after a predetermined operating time. In that context, the operating time is advantageously chosen such that the replaced blades may still be repaired.
If the blades of a turbine are replaced, the turbine rotor blades, the turbine stator vanes and the guiding segments are replaced. In order to be able to ensure, in the context of such a replacement, that the original turbine power is maintained, the original radial blade gap width must be set also for the new turbine rotor blades. To that end, the blade tips of the new rotor blades are accordingly shortened, before they are mounted, by removal of material at the blade tip. Since the radial gap width depends not only on the manufacturing tolerances of the new rotor blades but also on other factors, the final radial blade gap results only after the completed replacement of all blades. Influential factors may for example be the contour of the new stator vanes, the contour of the new guiding rings, the coating of the new stator vanes and guiding rings, the change in position of the new blades relative to the position of the replaced old blades, the manufacturing tolerances of the casing, the rotor disk slots and the like. The gap width which is finally set is accordingly difficult to calculate in advance and this can be done only within a very broad tolerance range.
Shortening the blade tips of rotor blades is normally carried out within the framework of a grinding machining using a grinding disk.
When building new rotors, it is known to clamp the entire bladed rotor in a large grinding machine, whereupon the blades are ground entirely automatically to the required length with a straight grinding disk.
Within the context of maintenance work, it is known to remove the rotor of a turbomachine in a first step. In a further step, the rotor disks as a whole may then be clamped in a grinding apparatus. A high-speed rotor with a straight grinding disk is attached thereto on an arm which rotates about the blade tips. In that context, an automatic feed in two directions and manual travel are carried out.
It is further known to grind blades individually. To that end, the blades are clamped individually in a grinding apparatus which carries a pneumatic high-speed rotor with a straight grinding disk. The high-speed rotor oscillates about the blade tip and has, in addition to the oscillatory movement, an automatic feed and a manual travel.
Another alternative consists in milling the blade tips for shortening purposes. In the CNC milling machines used for this, the blades are clamped individually and are milled to the required length.
One disadvantage of the grinding apparatuses with a straight grinding disk, used to shorten rotor blades, is that on account of the limited width of the grinding disk, which is approximately 20 mm in mobile use, it is necessary to move the grinding disk along the contour of the blade tip. Accordingly, an automatic feed is necessary, which leads to a complex construction of the grinding apparatus and to high costs. Furthermore, a long machining time is associated with the automatic feed.
One disadvantage of the milling methods used to shorten rotor blades of a turbomachine is, on one hand, that these are very expensive, which is not least due to the high initial costs of the CNC milling machines used and the high tool costs. On the other hand, the milling methods are very slow. Thus, for example, in practice 12 layers at 10 hours are needed to machine four turbine stages, to name but one example.